Pitt, who began his career at Chelsea, has made over 100 appearances for Cambridge since joining them in 2005.
Neither player can appear against the other club in any competition and will not feature in this Saturday's Blue Square Premier game between the clubs.
Russell has spent time on loan at Tamworth after failing to secure a place in the first team this season.
Meanwhile, winger Craig Nelthorpe has moved to Luton Town on loan until the end of the year after his spell at Barrow.
York have also signed loan striker Kevin Gall on a permanent contract until the end of the season.
Gall, who has been at Kitkat Crescent since November, scored his first goal for the club in last weekend's 4-1 win over Hayes & Yeading.
Source: BBC Sport